Underground networks such as electrical distribution networks, telecommunications networks, cable television networks and the like, comprise structures which must sometimes be replaced. These structures that are buried in the ground can be manholes, access chambers, access vaults, junction box and others.
One of the reasons that forces to replace the existing structures is the premature deterioration of the material (concrete or bricks) used at the time of the original construction. Another reason that forces to replace the existing structures is to comply with new requirements established for reasons of maintenance, safety, ergonomy and/or addition of equipment.
In practice, putting a network out of order even for a temporary period of time is not acceptable because numerous residential and commercial subscribers (read “customers”) would be deprived of essential services. Any interruption of this type would prove to be extremely expensive for the subscribers. Some of these customers are hospitals, buildings with offices or shops which cannot interrupt their activities.
Consequently, it is necessary to develop working methods that would allow the people in charge of the maintenance of the networks to carry out different modifications while keeping the networks functional during the work.
An existing method currently used on building sites consists of demolishing the existing structure and rebuilding a new structure while maintaining in service the existing underground network. As an example, mention can be made of Hydro-Quebec which maintains its electrical supply underground cables functional in their original positions during all steps of demolition and reconstruction.
By way of example of how such an existing method can be carried out, reference can be made to the case where the structure to be replaced is an access vault (junction box) in which are located electrical cables, telecommunications cables as well as television cables. The cables are usually uncovered inside the access vault. However, these cables between an access vault to another one extend within poly vinyl chloride (PVC) conduits which are generally embedded with concrete (such are also called concrete duct).
In such a case, the existing method comprises two main steps, the first one being the demolition of the existing access vault, the second one being the reconstruction of such a vault.
During the demolition step, the following sub-steps are carried out:                excavating and removing the material that covers and/or surrounds the structure;        installing inside the structure, a self-bearing wooden roof under the ceiling of the structure (this roof is used to protect the equipment inside the structure during the demolition);        demolishing and removing the chimney of the structure;        demolishing and removing the ceiling of the structure;        installing a horizontal beam made of steel and/or of wood on the surface of the street directly on top of the structure in the same direction as the existing cables;        installing nylon cords attached to the beam for supporting the cables;        demolishing and removing the vertical walls and the floor of the structure; and        demolishing and removing the concrete of the duct on a few meters on each side of the structure to give some flexibility to move the cables in a safe manner while the workers are still working.        
During the reconstruction step, the following sub-steps are carried out:                excavating the ground to fit the dimensions of the new structure;        preparing the foundation of the new structure;        assembling and levelling a framing for the floor;        pouring concrete to form the floor;        assembling a framing to form the vertical walls. During this sub-step, the contractor must girdle the existing cables. To do so, wood boards must be cut to form semi-circle openings which are then positioned on opposite sides of each existing cable. Also, bell shaped PVC components must be cut and placed on both sides of each cable. These bell shaped PVC components offer a soft surface for pulling the cables. This sub-step requires a great dexterity from the workers because they handle nails and hammers near electric cables under tension;        installing PVC conduits around the existing cables, these conduits extending from the structure up to the still intact concrete duct, (conduits cut in half over their length are used to carry out this task);        pouring concrete to form the vertical walls;        preparing a framing for the concrete duct;        pouring concrete to form the concrete duct;        installing a framing to form a new roof into the new structure. To carry out this sub-step, the contractor must install beams resting on the floor of the structure to support the framing in which the concrete will be poured to form the new roof. These beams have to be installed through the existing electrical cables without touching or damaging them;        pouring concrete to form the roof;        assembling a framing for chimneys;        pouring concrete to form the chimneys;        letting the concrete harden;        installing embankment material;        assembling a framing for a sidewalk;        pouring concrete to form the sidewalk and the asphalt for the paving; and        restarting public services (sidewalk, circulation lane, etc).        
As may be noticed, the existing method described hereinabove requires a long time intervention of the workers, which increases the risk of injuries, particularly when the workers are close to electrical cables under tension. Moreover, the total time needed to complete the work is very long. Indeed, the time required to carry out all the steps mentioned above is approximately four weeks.
Thus, there is a need for a faster and safer method for replacing underground structures.